Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure from a piezoelectric element, a system of jetting out a liquid droplet by generating a bubble in the ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The ink composition used for such inkjet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks an aqueous ink is predominating in view of production, handleability, odor, safety and the like.
The coloring agent used in such an ink for inkjet recording is required to have high solubility in a solvent, enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing property to an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. However, it in very difficult to find out a coloring agent satisfying these requirements in a high level. Various dyes and pigments for inkjet recording have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found out at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording. Heretofore, studies are being made on a dye having good color hue and fastness with an attempt to develop a dye excellent as the coloring agent for inkjet recording. However, in the compound called a water-soluble dye, a water-soluble group is necessarily substituted. If the number of water-soluble groups is increased so as to improve the stability of ink, this has been found to cause a problem that the formed image is readily blurred under high-humidity conditions.
The present inventors have found that a betaine compound is effective for solving this phenomenon of blurring.
However, it is revealed that when an ink set contains an ink using a betaine compound, the bleeding can be improved depending on the surfactant contained in other inks, but the coloring material is precipitated in the high-density part having mixing of colors and this readily causes production of an image with bronze gloss or occurrence of a beading-phenomenon.